The water absorbent resin has been hitherto used as one component for hygienic materials such as sanitary cotton, disposable diaper, and absorbents for other kinds of body fluid. As concrete examples of the water absorbent resin, hydrolyzate of starch-acrylonitrile graft polymer, neutralized starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic acid ester copolymer, hydrolyzate of acrylonitrile copolymer or acrylamide copolymer, and the product of cross-linkage thereof, and partially neutralized cross-linked acrylic acid may be cited. These water absorbent resins invariably possess an internal cross-linked structure and exhibit no solubility in water.
The characteristic properties which these water absorbent resins are expected to possess include high absorption capacity against no pressure and against pressure, excellent absorption speed, high gel strength, and fully satisfactory suction force necessary for sucking water from a medium, for example. Since the water absorbing properties are affected by crosslink density, they do not necessarily manifest positive correlations with one another as evinced by the fact that an increase in the crosslink density leads to an increase in the gel strength but a decrease in the amount of water absorbed. Particularly, the absorption capacity is in a contradictory relation with the absorption speed, the gel strength, and the suction force, for example. The water absorbent resin which has acquired an enhanced absorption capacity, therefore, possibly shuns uniform absorption of water and forms portions of partial aggregation of itself when the water absorbent resin particles contact with water and induces extreme degradation of the absorption speed because the water is not diffused throughout the entire volumes of water absorbent resin particles.
For the purpose of relaxing this phenomenon and obtaining a water absorbent resin which has a high absorption capacity and a comparatively satisfactory absorption speed, a method for giving the water absorbent resin particles a surface coated with a surfactant or a nonvolatile hydrocarbon has been available. This method indeed exalts the dispersibility of the initially absorbed water but brings no sufficient effects in enhancing the absorption speed and the suction force of the individual resin particles.
As a means to produce a polyacrylic acid type polymer of high water absorbing property, a method which comprises causing an aqueous composition having a partial alkali metal salt of polyacrylic acid as a main component and having a low crosslink density to be heated in the presence of a water-soluble peroxide radical initiating agent thereby introducing a crosslink therein by radical cross-linkage has been proposed (U.S. Pat. No. 4,910,250). It is difficult to distribute uniformly internal cross-links in the polymer and uneasy to adjust the crosslink density. Thus, a measure of preparing a polymer which contains water-soluble polyacrylic acid gel having low crosslink density and then heating the polymer together with a persulfate added thereto as a polymerization initiator is adopted. Patent Document 1 claims to realize precise control of crosslink density by adjusting the amount of the initializing agent to be added and, owing to the uniform presence of crosslink in the polymer, acquire perfect water absorbing properties and obtain as well a water absorbent resin devoid of stickiness.
While the persulfate which is used in the Patent Document 1 mentioned above is decomposed by heat, it is decomposed by ultraviolet rays and generates radicals (J. Phys. Chem., 1975, 79, 2693, and J. Photochem. Photobiol., A, 1988, 44, 243). Since the persulfate fulfills a function as a polymerization initiator, the aqueous solution of a water-soluble vinyl monomer, when exposed to radiation, undergoes polymerization and radical cross-linkage simultaneously and produces a hydrogel (JP-A-2004-99789). A reaction system which forms an internal crosslink by adding a hydrophilic polymer component, a photo-polymerization initiator, and a cross-linking agent together and irradiating them with ultraviolet rays has been known (WO 2004/031253).
Meanwhile, a method which gives a water absorbent resin a surface treatment with a cross-linking agent and imparts thereto a surface of a heightened crosslink density has been also known (U.S. Pat. Nos. 4,666,983 and 5,422,405, for example). Such water absorbent resins as cited in the preceding patent documents entail the presence of a reactive functional group on their surfaces. By effecting introduction of a crosslink between functional groups in consequence of the addition of a surface cross-linking agent capable of reacting with the functional groups, it is made possible to give to the water absorbent resin a surface of increased crosslink density and enable the water absorbent resin to acquire water absorbing properties perfect even under pressure.
Further, since the use of the surface cross-linking agent mentioned above requires the reaction for the formation of cross-links to be performed at a high temperature for a long time and entails the problem of suffering persistence of the cross-linking agent in the unaltered state, a method which, by causing an aqueous solution containing a peroxide radical initiating agent to contact a resin and heating the resin, accomplishes introduction of cross-links into polymer molecular chains in the neighborhood of the surface of the resin by virtue of decomposition of the radical initiating agent has been proposed (U.S. Pat. No. 4,783,510). In a working example of this method, a water absorbent resin exhibiting an exalted absorption capacity was obtained by affecting the heating with superheated steam at 130° C. for 6 minutes.
Further, JP-A-2005-97585 discloses a technique for modifying the surface of the water absorbing resin and enhancing the absorbency against pressure by adding a processing liquid containing a radical polymerizing compound and a particulate water absorbing resin and irradiating the resultant mixture with active energy rays. Since this method uses a radical polymerizing compound, however, it has greatly lowered the absorbency against no pressure and entailed a very high cost.
JP-A-63-260907 discloses a technique for decreasing residual monomer content in a water absorbing resin by irradiating a water absorbing resin having specific water content with ultraviolet rays without adding a radical polymerizing compound. However, this technique does not involve the flow of the water absorbing resin during the irradiation with ultraviolet rays, therefore the modification of the surface of the water absorbing resin was performed extremely unevenly, if any.